Step index optical fiber with doped cladding and core, a preform, and a method of fabricating such a fiber

ABSTRACT

The invention provides a step index optical fiber which presents cladding having an index lower than the index of silica and a core with an index higher than the index of silica. The fiber is obtained by drawing a preform made by chemical vapor deposition using a deposition tube of index lower than the index of silica. Inner cladding of index substantially equal to the index of the deposition tube, and then a core of index higher than the index of the inner cladding are deposited in succession therein. The invention makes it possible to obtain a fiber having a large effective area, reduced attenuation, and suitable for being fabricated at low cost by chemical vapor deposition.

[0001] The present invention relates to the field of optical fibertransmission, and more specifically to step index optical fibers used asline fibers in such transmission systems.

BACKGROUND OF THE INVENTION

[0002] The index profile of an optical fiber is generally described interms of the appearance of the graph plotting the refractive index ofthe fiber as a function of radius. In conventional manner, the distancer to the center of the fiber is plotted along the abscissa axis and thedifference between the refractive index and the refractive index of thefiber cladding is plotted up the ordinate axis. The index profile canthus be said to be “stepped”, “trapezium-shaped”, or “triangular” forgraphs that are respectively step-, trapezium-, or triangle-shaped. Suchcurves are generally idealized profiles or reference profiles for thefiber, and fiber fabrication constraints can lead to a profile thatdeparts perceptibly therefrom.

[0003] It is conventional for the line fiber in optical fibertransmission systems to be a step index fiber, also referred to as asingle mode fiber (SMF). The Applicant company thus sells a single modestep index fiber under the reference ASMF 200 which presents a chromaticdispersion canceling wavelength λ₀ in the range 1300 nanometers (nm) to1320 nm, and chromatic dispersion that is less than or equal to 3.5picoseconds per nanometer kilometer (ps/(nm.km)) in the range 1285 nm to1330 nm, and of 17 ps/(nm.km) at 1550 nm. At 1550 nm the chromaticdispersion slope is about 0.06 ps/(nm².km).

[0004] WO-A-00 36443 describes a step index optical fiber presenting acore of index greater than that of the cladding. The cladding issurrounded in a layer of carbon. The core presents a diameter lying inthe range 9.5 micrometers (μm) to 12.0 μm. The relative difference inindex between the core and the cladding lies in the range 0.3% to 0.5%.In that document, it is stated that the fiber can be fabricated usingsilica, by doping the core with germanium and using silica cladding; analternative is to dope the cladding with fluorine, while using a silicacore.

[0005] Both of those solutions suffer from drawbacks. Firstly, dopingthe core with germanium requires germanium to be used at highconcentration—typically greater than 5% by weight—in order to achievethe required index difference relative to silica cladding. Suchgermanium concentrations increase attenuation in the fiber. Furthermore,fabricating a silica-core fiber with doped cladding, as suggested inthat document, implies using a vapor axial deposition (VAD) techniqueduring fabrication, or else using an outside vapor phase oxidation(OVPO) technique. WO-A-00 42458 describes a transmission fiber forlong-distance transmission systems; the cladding is fluorine-doped andthe core is chlorine-doped. The fiber is fabricated using a VADtechnique.

[0006] To manufacture optical fibers, the modified chemical vapordeposition (MCVD) technique is also used. Layers of silica containingdopant for varying its index are deposited successively inside adeposition tube. Thereafter the tube is collapsed or contracted so as toconstitute a first preform. This first preform is inserted in one ormore sleeves which are collapsed or contracted in turn so as to pressagainst the first preform. The resulting preform is drawn to form afiber. Such techniques for fabricating a fiber are well known to theperson skilled in the art.

[0007] Thus, EP-A-0 972 752 describes MCVD fabrication and it proposesdepositing successive layers of cladding material and of core materialinside a deposition tube. After contraction, the deposition tube isinserted in one or more sleeves; it is proposed that the inner sleeveshould present doping to lower its index so as to constitute aburied-cladding fiber. The cladding deposited inside the deposition tubecan be doped with fluorine, the core being doped with germanium.Providing the purity of the deposition tube is sufficient, it ispossible to avoid depositing cladding. The deposition tube is a glasstube in which the concentration of OH⁻ ions is less than 0.05 parts permillion (ppm) by weight. U.S. Pat. No. 4,566,754 or U.S. Pat. No.5,692,087 thus propose a step index fiber manufactured by MCVD, in whichfluorine-doped cladding and a germanium-doped core are deposited insidea silica deposition tube.

[0008] For the same type of preform, U.S. Pat. No. 5,942,296 suggestsfacilitating drawing down the preform by acting on the viscosity and thethermal conductivity of the silica deposition tube and of the sleeve(s)surrounding it. That solution makes it possible to avoid heating thecore of the preform. It is specified in that document that the cost ofMCVD fabrication decreases with decreasing thickness of thefluorine-doped cladding.

[0009] EP-A-0 899 243 also proposes a step index fiber presenting agermanium-doped core, inner cladding that is fluorine-doped, and outercladding of non-doped silica. That application proposes drawing thefiber at a speed greater than 20 grams per minute (g/min).

[0010] EP-A-0 863 108 describes a method of fabricating a preform byplasma deposition of build-out material on the outside of the depositiontube.

OBJECTS AND SUMMARY OF THE INVENTION

[0011] The problem of the invention is that of fabricating step indexfibers by MCVD. Compared with the solution proposed in those documents—asilica deposition tube, fluorine-doped cladding, and a germanium-dopedcore—the invention makes it possible to simplify the preform fabricationprocess and to reduce its duration and its cost. The invention alsomakes it possible to obtain a step index fiber having an effective areathat is increased and attenuation that is decreased.

[0012] More precisely, the invention provides a method of fabricating apreform for drawing into an optical fiber, the method comprising:

[0013] using chemical vapor deposition to form inner cladding inside adeposition tube, the deposition tube presenting a refractive index lowerthan that of silica and the inner cladding presenting an indexsubstantially equal to the index of the deposition tube, the relativedifference between the index of the inner cladding and the index of thedeposition tube having an absolute value smaller than 0.02%; and

[0014] using chemical vapor deposition to form a core inside thecladding, the core presenting an index higher than the index of silica.

[0015] In an implementation, the difference between the index of thedeposition tube and the index of silica is less than −0.5×10⁻³.

[0016] In another implementation, the difference between the index ofthe core and the index of the inner cladding lies in the range 4.3×10⁻³and 5×10⁻³.

[0017] The method may further comprise:

[0018] collapsing the deposition tube with the inner cladding and thecore; and

[0019] depositing build-out material around the collapsed tube, thebuild-out material presenting an index substantially equal to the indexof the deposition tube.

[0020] It is also possible to provide for:

[0021] collapsing the deposition tube with the inner cladding and thecore; and

[0022] collapsing a sleeve around the collapsed tube, the sleevepresenting an index substantially equal to the index of the depositiontube.

[0023] The invention also provides a preform for drawing into an opticalfiber, the preform comprising:

[0024] a deposition tube presenting an index lower than the index ofsilica;

[0025] cladding inside the deposition tube presenting an indexsubstantially equal to the index of the deposition tube, the relativedifference between the index of the inner cladding and the index of thedeposition tube having an absolute value of less than 0.02%; and

[0026] a core inside the cladding presenting an index greater than theindex of silica.

[0027] It is advantageous for the difference between the index of thedeposition tube and the index of silica to be less than −0.5×10⁻³.Advantageously, the difference between the index of the core and theindex of the inner cladding lies in the range 4.3×10⁻³ and 5×10⁻³.

[0028] Build-out material can be provided around the deposition tube,which material presents a refractive index substantially equal to theindex of the deposition tube, or else a sleeve can be providedpresenting an index that is substantially equal to the index of thedeposition tube.

[0029] The invention also provides a method of fabricating an opticalfiber, comprising drawing a fiber from such a preform.

[0030] Finally, the invention proposes an optical fiber comprising:

[0031] cladding having an index lower than the index of silica;

[0032] inner cladding extending inside said cladding and presenting anindex substantially equal to the index of the cladding, the relativeindex difference between the index of the inner cladding and the indexof the cladding having an absolute value smaller than 0.02%; and

[0033] a core extending inside the inner cladding and presenting anindex higher than the index of silica.

[0034] In an embodiment, the fiber presents:

[0035] a difference Δn between the index of the core and the index ofthe inner cladding lying in the range 4.3×10⁻³ and 5×10⁻³;

[0036] a core radius r₁ lying in the range 4.8 μm to 5.7 μm; and

[0037] a product r₁×{square root}Δn of the core radius multiplied by thesquare root of said difference lying in the range 0.336 μm to 0.378 μm.

[0038] The optical fiber may present cladding whose index differencerelative to the refractive index of silica is less than −0.5×10⁻³ and arelative index difference between the index of the inner cladding andthe index of the cladding smaller than 0.02% in absolute value.

[0039] The inner cladding is characterized by quasi-periodic variationsof index lying in the range 0.3×10⁻³ to 0.5×10⁻³ in absolute value,presenting symmetry about the axis of the fiber with periodicity ofabout 0.6 μm to 1.4 μm. This inner cladding can be doped using C₂F₆, inwhich case the rate at which the index varies as given by the ratiobetween the peak-to-peak difference in the index and the half-period ofthe variations lies in the range 0.4×10⁻³ μm⁻¹ and 1.7×10⁻³ μm⁻¹. Theinner cladding can also be doped with SiF₄, in which case the periodremains unchanged and the ratio between the absolute peak-to-peak indexdifference and the half-period of the variations lies in the range0.2×10⁻³ μm⁻¹ and 0.7×10⁻³ μm⁻¹.

[0040] It is also advantageous for the rate of index variation in thecladding to be less than 0.1×10⁻³ μm⁻¹.

[0041] The fiber can also present an effective area greater than orequal to 90 μm² at the wavelength of 1.55 μm, or a theoretical cutoffwavelength that is less than or equal to 1.65 μm.

[0042] In terms of profile, the optical fiber can be characterized byone or more of the following relationships:

[0043] the ratio between the radius r₂ (in μm) of the deposition tubeand the radius r₁ (in μm) of the core is greater than or equal to−7.33×r₁×{square root}Δn+4.36, where r₁×{square root}Δn is the productof multiplying the radius of the core by the square root of thedifference between the core index and the inner cladding index;

[0044] the ratio between the radius r₂ of the deposition tube and theradius r₁ of the core is less than or equal to −10.71×r₁×{squareroot}Δn+6.7, where r₁×{square root}Δn is the product of the core radiusmultiplied by the square root of the difference between the core indexand the inner cladding index.

[0045] It is also advantageous for the power propagating in the portionof the fiber that corresponds to the deposition tube to lie in the range0.025% to 1.4%.

BRIEF DESCRIPTION OF THE DRAWING

[0046] Other characteristics and advantages of the invention will appearon reading the following description of embodiments of the inventiongiven by way of example, and with reference to the accompanying drawingin which the sole FIGURE is a diagram showing the index profile of afiber of the invention.

MORE DETAILED DESCRIPTION

[0047] In order to fabricate a step index optical fiber by MCVD, theinvention proposes using a deposition tube with an index-loweringdopant; doped inner cladding is deposited inside the tube, whichcladding presents substantially the same refractive index as thedeposition tube; and then a doped core is deposited presenting arefractive index that is higher than the index of the cladding and ofthe deposition tube. After collapsing, it is also advantageous toprovide build-out material which is also doped to lower its index downto a value close to the index of the deposition tube.

[0048] The fiber obtained after drawing such a preform made using thismethod presents a core, inner cladding which corresponds to the dopedinner cladding as deposited inside the deposition tube, and claddingwhich corresponds to the deposition tube; in the fiber obtained in thisway, it is possible to distinguish between the cladding, the innercladding, and the core of the fiber. The cladding comes from thedeposition tube and it presents an index that is substantially constant;in any event, the cladding has no reason to present substantiallyperiodic variations in index that are circularly symmetrical about theaxis of the fiber. Similarly, the build-out material or the sleeve ofindex that does not exceed the index of the cladding by more than0.5×10⁻³ does not present any substantially periodic variation in indexeither. In contrast, the inner cladding results from chemical vapordeposition (CVD) inside the deposition tube. Insofar as such depositionis performed as a plurality of passes, the inner cladding presents aplurality of interfaces or small-amplitude index ripples. It istherefore possible to determine on a given optical fiber whether or notthere exists both cladding and inner cladding.

[0049] The level of bending losses is an important characteristic fortaking into consideration when making an optical fiber. In a step indexfiber, bending losses depend on the index of the cladding and on theratio r₂/r₁ which specifies the ratio of the radius r₂ marking thebeginning of the cladding as measured from the axis of the fiber to theradius of the core r₁. For a given cladding index, bending lossesincrease with decreasing ratio r₂/r₁. Similarly, for given ratio r₂/r₁,bending losses increase with cladding index. In order to maintain a lowlevel of bending losses, a level of less than 10⁻⁵ decibels per meter(dB/m) for a bending radius of 30 mm—the radius r₂ must remain greaterthan a minimum value r_(2m) which increases with increasing claddingindex. Assuming that the cladding index remains greater than or equal tothe index of the inner cladding, the value of r_(2m) is at a minimumwhen the index of the cladding is equal to the index of the innercladding. This applies in particular to fibers of the invention wherethe index of the deposition tube and of the build-out material issubstantially equal to the index of the inner cladding. In contrast,this does not apply to pure silica core fibers (PSCF) made using OVD orAVD techniques in which the index of the silica cladding is equal to theindex of the core. PSCFs are thus characterized by large values for theradius r₂ as defined above and consequently they require a largedeposition section for the inner cladding in order to maintain a lowlevel of bending losses. Compared with PSCFs, the solution of theinvention has the advantage of presenting a cladding index and abuild-out material index that are substantially equal to the index ofthe inner cladding, thus making it possible to limit to a considerableextent the thickness of the inner cladding, and thereby achieving acomparable reduction in fabrication cost. With fibers of the invention,the role of the inner cladding is mainly that of reducing thecontribution of the deposition tube to spectrum attenuation, whichcontribution is proportional to the power of the optical fieldpropagating in the tube and to the absorbance of the tube at theoperating wavelength.

[0050] The invention presents another advantage concerning spectrumattenuation. The small proportion of germanium dopant present in thedeposited layers of the core limits Rayleigh diffusion in the fibercore, and consequently limits spectrum attenuation. Using Δn_(c) todenote the difference between the index n_(c) of the core and the indexn_(g) of the cladding, the following equation can be written:

Δn _(c) =n _(c) −n _(g)=(n _(c) −n _(si))+(n _(si) −n _(g))  (1)

[0051] where:

[0052] (n_(g)<n_(si)); and

[0053] n_(si) designates the refractive index of silica.

[0054] The quantity of germanium to be deposited in the core of a fiberof the invention is proportional to (n_(c)−n_(si)) and not to(n_(c)−n_(g)) which shows the advantage of using cladding doped influorine whose index is lower than that of silica.

[0055] There follows an example of implementing a preform and a fiber ofthe invention using a modified chemical vapor deposition technique. Adeposition tube is used that has been doped to present an index that islower than that of silica: the doping can be fluorine doping, at aconcentration lying in the range 2500 ppm to 3500 ppm by weight, thushaving the effect of lowering the index of the tube to values below theindex of silica by at least 0.5×10⁻³. The deposition tube typicallypresents an inside diameter of 34 mm, an outside diameter of 39 mm, anda length lying in the range 1300 mm to 1690 mm. It is possible to usethe deposition tube sold under the reference F320 HERAEUS.

[0056] Inner cladding which is doped to lower its index relative to theindex of silica is formed on the inside of the deposition tube. Theinner cladding presents an index close to the index of the depositiontube. It is advantageous for the inner cladding to present an index thatis equal to that of the deposition tube; an index difference between theinner cladding and the deposition tube is acceptable providing itremains in the range −0.3×10⁻³ and 0. In relative terms, it isadvantageous for the relative difference between the index of the innercladding and the index of the deposition tube to be smaller than 0.02%in absolute value. Such differences have limited incidence on thetransmission properties of the resulting optical fiber. In particular,it is possible to use fluorine doping at a concentration of 0.3% to 1%by weight of fluorine in order to lower the index of the cladding. Inorder to form this inner cladding, conventional chemical vapordeposition techniques are used.

[0057] Thereafter, a core is formed inside the deposition tube, whichcore is doped so as to increase its index compared with the index ofsilica. Advantageously, the difference between the index of the core andthe index of the inner cladding lies in the range 4.3×10⁻³ to 5×10⁻³. Byway of example, these values correspond to index differences lying inthe range 3.3×10⁻³ to 4×10⁻³ relative to the index of silica when theindex difference between the inner cladding and the index of silica is−1×10⁻³.

[0058] After the inner cladding and the core have been deposited insidethe deposition tube, the preform is collapsed so as to close the openingthrough which gases pass inside the deposition tube. After thedeposition tube has been collapsed, a sleeve can be placed around it orbuild-out material can be placed on it, in conventional manner: a sleeveis collapsed around the preform whereas build-out material is depositedusing chemical vapor deposition or plasma deposition. The sleeve has arefractive index that is substantially constant, without any ripple. Thebuilt-out material can present ripple, depending where appropriate onthe fabrication process, for example variations in dopant. It isadvantageous for the index of the sleeve or of the built-out material tobe as close as possible to the index of the deposition tube. The sleeveor the build-out material can be doped, e.g. using fluorine. As for theinner cladding, a difference of less than 0.3×10⁻³ between the index ofthe sleeve or of the built-out material and the index of the depositiontube is acceptable.

[0059] The dimensions of the preform can be as follows:

[0060] outside diameter of the core: 3.5 mm to 4 mm;

[0061] inside diameter of the deposition tube: 8 mm to 10 mm;

[0062] outside diameter of the deposition tube: 18 mm to 22 mm; and

[0063] final diameter of the built-out or sleeved preform: 39 mm to 45mm.

[0064] Such a preform can be drawn using a conventional fiber-drawingmethod to obtain a fiber presenting the reference index profile shown inFIG. 1. Distance r to the center of the fiber is plotted along theabscissa axis, and difference between refractive index and therefractive index of the cladding of the fiber is plotted up the ordinateaxis. The profile shown in FIG. 1 is a step index profile. There can beseen around the axis of the fiber a fiber core which presents an indexgreater than that of silica; the core radius r₁ lies in the range 4.8 μmto 5.7 μm.

[0065] Around the core, the fiber presents an index that issubstantially constant and that is less than that of silica. Between theradii r₁ and r₂ there extends the portion of the fiber which correspondsto the inner cladding; r₂ typically lies in the range 12.5 μm to 15.7μm. This portion of the fiber is referred to below as the “innercladding”. As explained above, it differs from the portion of the fiberwhich corresponds to the deposition tube because of the presence ofripple in its index. This ripple constitutes quasi-periodic variationsin the value of the index along the radius of the index profile, i.e.along a radius of the fiber. Its period depends on the thickness of thelayers deposited inside the deposition tube, and on the way dimensionsvary when the preform is collapsed and subsequently while it is beingdrawn. These quasi-periodic variations can depend on the dopants used.C₂F₆ can be used as the dopant for inner cladding indices lowered to−1.5×10⁻³ relative to the index of silica. It is common practice to useSiF₄ to reach index values of less than −1.5×10⁻³ relative to the indexof silica. The relative peak-to-peak size of the index ripple can varydepending on the dopant used. Typical values for variations with C₂F₆ asdopant lie in the range 0.3×10⁻³ to 0.5×10⁻³. Given the quasi-periodicnature of the ripple, the ratio between this peak-to-peak variation andthe half-period lies in the range 0.4×10⁻³ μm⁻¹ and 1.7×10⁻³ μm⁻¹.Typical values for variations when using SiF₄ as the dopant are smaller,being situated typically around 0.2×10⁻³, which leads to a ratio forpeak-to-peak variations to the half-period lying in the range 0.2×10⁻³μm⁻¹ to 0.7×10⁻³ μm⁻¹. It is also possible for the dopant in the innercladding to be constituted by germanium at a concentration of 0.3% to 1%by weight and/or phosphorus at a concentration of 0.3% to 0.4% byweight. Where necessary, germanium makes it possible to limit spectrumattenuation, while phosphorus provides for better vitrification of thedeposit.

[0066] Beyond the radius r₂, there extends the portion of the fiberwhich corresponds to the deposition tube and to the build-out. Theportion corresponding to the deposition tube is referred to below as the“cladding”. Variations in the index of the cladding are associated withthe uniformity of the index in the deposition tube. In the cladding,index generally varies slowly and these variations generally remainsmaller than 0.1×10⁻³. In contrast, unlike the inner cladding, thesevariations are not periodic. The rate of vibration, measured radiallyover a distance of 1 μm, is smaller than 0.1×10⁻³ μm⁻¹, in other wordsis smaller than the radial index variations in the inner cladding.

[0067] The index difference between the core of the fiber and the innercladding, i.e. the index step in the fiber, typically lies in the range4.3×10⁻³ to 5×10⁻³.

[0068] The fiber of the invention presents the following propagationcharacteristics, as measured at 1550 nm:

[0069] chromatic dispersion: in the range 18.7 ps/(nm.km) to 20.3ps/(nm.km);

[0070] chromatic dispersion slope: in the range 0.058 ps/(nm².km) to0.61 ps/(nm².km) theoretical cutoff wavelength: less than or equal to1650 nm;

[0071] bending losses for 10 mm: less than or equal to 15 dB/m;

[0072] effective area: greater than or equal to 90 μm²;

[0073] mode diameter: greater than or equal to 10.7 μm; and

[0074] attenuation: less than or equal to 0.22 dB/km.

[0075] Bending losses are measured by winding the fiber around a sleevehaving a diameter of 10 mm.

[0076] By way of example, a fiber having a core radius of 5.7 μm,presenting inner cladding having an outside radius of 13.68 μm, with anindex that is 1.2×10⁻³ below the index of silica and an index step of4.4×10⁻³ between the inner cladding and the core has the followingpropagation characteristics at 1550 nm:

[0077] chromatic dispersion: 19.6 ps/(nm.km);

[0078] chromatic dispersion slope: 0.06 ps/(nm².km);

[0079] theoretical cutoff wavelength: 1645 nm;

[0080] bending losses: 12.3 dB/m;

[0081] effective area: 113 μm²;

[0082] mode diameter: 11.8 μm; and

[0083] attenuation: 0.18 dB/km.

[0084] The invention makes it possible to use MCVD to obtain a stepindex fiber presenting a large effective area and low attenuation. Theattenuation values obtained result in particular from the dopantconcentration in the fiber core, and this is lower than instate-of-the-art fibers. The method of the invention makes it possibleto fabricate at lower cost because of the reduced thickness of the innercladding that needs to be deposited inside the deposition tube.

[0085] The fiber of the invention can be used for any known applicationof step index fibers, and in particular as a line fiber in an opticalfiber transmission system. It generally presents the followingpropagation characteristics. For a fluorine-doped tube having an indexthat is less than the index of silica by at least 0.5×10⁻³, and for thecore having an index value lying in the range 4×10⁻³ to 5×10⁻³ relativeto the tube index, the values of the index step relative to the index ofsilica remain less than 4.5×10⁻³. As explained above, this value is lessthan the value obtained for a conventional silica-clad step index fiber;this reduction in the index of the core relative to the index of silicain the fiber of the invention corresponds to reducing Rayleigh diffusionin the fiber.

[0086] The profiles obtained when the conditions of the precedingparagraph are satisfied comply with the following inequalities:

[0087] 4.3×10⁻³≦Δn≦5×10⁻³ where Δn is the difference between the coreindex and the inner cladding index;

[0088] 4.8 μm≦r₁≦5.7 μm where r₁ is the radius of the fiber core; and

[0089] 0.336 μm≦r₁×{square root}Δn≦0.378 μm.

[0090] The third relationship is one possible solution for ensuring thatthe fibers have an effective area that is greater than or equal to 90μm² and less than or equal to 115 μm², with a theoretical cutoffwavelength of less than 1.65 μm.

[0091] It is also possible to determine the extreme positions for thedeposition tube as a function of radius and for index; for this purpose,it is possible to use the following relationships:

(r ₂ /r ₁)min=−7.33×r ₁ ×{square root}Δn+4.36

[0092] and

(r ₂ /r ₁)max=−10.71×r ₁ ×{square root}Δn+6.7

[0093] When the above relationships are satisfied, the power of the modepropagating in the deposition tube lies in the range 0.025% to 1.4% ofthe total power propagating in the fiber, and the attenuation incrementdue to absorption by the deposition tube remains less than about 0.01dB/km.

[0094] In the above description, the details of implementing modifiedchemical vapor deposition are not described since they are well known tothe person skilled in the art. The invention is described in the contextof MCVD. It could also be implemented using plasma-assisted chemicalvapor deposition (PCVD), or any other technique for depositing inside adeposition tube. The invention also makes it possible to obtain a fiberpresenting a different profile.

1/ A method of fabricating a preform for drawing into an optical fiber,the method comprising: using chemical vapor deposition to form innercladding inside a deposition tube, the deposition tube presenting arefractive index lower than that of silica and the inner claddingpresenting an index substantially equal to the index of the depositiontube, the relative difference between the index of the inner claddingand the index of the deposition tube having an absolute value smallerthan 0.02%; and using chemical vapor deposition to form a core insidethe cladding, the core presenting an index higher than the index ofsilica. 2/ The method of claim 1, wherein the difference between theindex of the deposition tube and the index of silica is less than−0.5×10⁻³. 3/ The method of claim 1, wherein the difference between theindex of the core and the index of the inner cladding lies in the range4.3×10⁻³ and 5×10⁻³. 4/ The method of claim 1, further comprising:collapsing the deposition tube with the inner cladding and the core; anddepositing build-out material around the collapsed tube, the build-outmaterial presenting an index substantially equal to the index of thedeposition tube. 5/ The method of claim 1, further comprising:collapsing the deposition tube with the inner cladding and the core; andcollapsing a sleeve around the collapsed tube, the sleeve presenting anindex substantially equal to the index of the deposition tube. 6/ Apreform for drawing an optical fiber, the preform comprising: adeposition tube presenting an index less than the index of silica;cladding inside the deposition tube presenting an index substantiallyequal to the index of the deposition tube, the relative differencebetween the index of the inner cladding and the index of the depositiontube having an absolute value smaller than 0.02%; and a core inside thecladding presenting an index higher than the index of silica. 7/ Thepreform of claim 6, wherein the difference between the index of thedeposition tube and the index of silica is less than −0.5×10⁻³. 8/ Thepreform of claim 6, wherein the difference between the index of the coreand the index of the inner cladding lies in the range 4.3×10⁻³ and5×10⁻³. 9/ The preform of claim 6, having build-out material around thedeposition tube, the build-out material having an index substantiallyequal to the index to the index of the deposition tube. 10/ The preformof claim 6, having a sleeve around the deposition tube, the sleevepresenting an index substantially equal to the index of the depositiontube. 11/ A method of fabricating an optical fiber, comprising drawing apreform according to claim
 6. 12/ An optical fiber presenting: claddinghaving an index less than the index of silica; inner cladding extendinginside said cladding and presenting an index substantially equal to theindex of the cladding, the relative index difference between the indexof the inner cladding and the index of the cladding having an absolutevalue of less than 0.02%; and a core extending inside the inner claddingand presenting an index greater than the index of silica. 13/ An opticalfiber according to claim 12, presenting: a difference Δn between theindex of the core and the index of the inner cladding lying in the range4.3×10⁻³ and 5×10⁻³; a core radius r₁ lying in the range 4.8 μm to 5.7μm; and a product r₁×{square root}Δn of the core radius multiplied bythe square root of said difference lying in the range 0.336 μm to 0.378μm. 14/ The optical fiber of claim 12, presenting cladding in which theindex difference relative to the index of silica is less than −0.5×10⁻³.15/ The optical fiber of claim 12, wherein the inner cladding presentsquasi-periodic variations in index that are symmetrical about the axisof the fiber. 16/ The optical fiber of claim 15, wherein the innercladding is doped using C₂F₆, and wherein the ratio between the absolutepeak-to-peak difference in the index and the half-period of the variantslies in the range 0.4×10⁻³ μm⁻¹ to 1.7×10⁻³ μm⁻¹. 17/ The optical fiberof claim 15, wherein the inner cladding is doped using SiF₄. and whereinthe ratio between the peak-to-peak index difference and the half-periodof the variants lies in the range 0.2×10⁻³ μm⁻¹ to 0.7×10⁻³ μm⁻¹. 18/The optical fiber of claim 12, wherein the rate of index variation inthe cladding as measured radially over a distance of 1 μm is less than0.1×10⁻³. 19/ The optical fiber of claim 12, presenting an effectivearea greater than or equal to 90 μm² at the wavelength of 1.55 μm. 20/The optical fiber of claim 12, presenting a theoretical cutoffwavelength that is less than or equal to 1.65 μm. 21/ The optical fiberof claim 12, wherein the ratio between the radius r₂ of the depositiontube and the radius r₁ of the core is greater than or equal to:−7.33×r₁×{square root}Δn+4.36 where r₁×{square root}Δn is the product ofthe core radius multiplied by the square root of the difference betweenthe core index and the inner cladding index. 22/ The optical fiber ofclaim 12, wherein the ratio between the radius r₂ of the deposition tubeand the radius r₁ of the core is less than or equal to:−10.71×r₁×{square root}Δn+6.7 where r₁Δ{square root}Δn is the product ofthe core radius multiplied by the square root of the difference betweenthe core index and the inner cladding index. 23/ The optical fiber ofclaim 12, wherein the power of the mode propagating in the portion ofthe fiber corresponding to the deposition tube lies in the range 0.025%to 1.4%.